fix: pass null pointer for jemalloc prof.dump

[kixelated]

Summary

• jemalloc's prof.dump mallctl expects a NULL pointer to use the prof_prefix path from MALLOC_CONF
• The previous code passed an empty C string (b"\0"), which jemalloc treated as an invalid filename, causing dumps to fail with EFAULT
• Confirmed on usc.cdn.moq.dev — profiling is active but kill -USR1 produces: failed to dump heap profile err=An interface with side effects failed

Test plan

• Deploy to a relay node and send SIGUSR1
• Verify heap dump files appear at /tmp/moq-relay.heap.<pid>.<seq>.heap

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[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 0af981c5-e30e-46f7-9073-93a6c92de4b7

📥 Commits

Reviewing files that changed from the base of the PR and between 373e6da and 3913ab6.

📒 Files selected for processing (1)

• rs/moq-relay/src/jemalloc.rs

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Walkthrough

The jemalloc module in the moq-relay crate updates the heap-profile dump trigger mechanism. The change modifies the second argument passed to raw::write when invoking prof.dump, replacing a cast empty byte-string literal with a typed null pointer. The surrounding signal handling logic, match-based success and error logging behavior, and overall control flow remain unchanged.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: replacing an empty C string with a null pointer for jemalloc's prof.dump mallctl argument.
Description check	✅ Passed	The description clearly explains the problem (jemalloc expecting NULL pointer, previous code passing empty string causing EFAULT), the observed failure, and the intended fix.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/jemalloc-prof-dump

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch fix/jemalloc-prof-dump

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